1. Field of the Invention
The present invention relates to analog filter circuits, and particularly to optimal low power complex filters.
2. Description of the Related Art
Active complex (polyphase) filters are renewed as they provide solutions for image rejection in low-IF wireless applications such as Bluetooth (BT) and Zigbee receivers. Also, they can be utilized in wireless sensor network and IEEE 802.15.4 applications such as gm-C complex filters known in the prior art. These filters are often based on transconductance amplifier-C (gm-C) or active-RC techniques. In addition, several polyphase filter realizations based on the second generation current conveyors (CCIIs), current feedback amplifiers (CFAs), current amplifiers (CAs) and current mirrors can, for example, be found in other prior arts. These filters can be classified based on their synthesis method into three categories: element substitution techniques of LC prototypes, cascading of first-order complex sections, and cascading of second-order complex biquads.
Complex filters based on LC simulation often use extensive number of active devices. For example, an exemplary prior art filter employs 30 transconductance amplifiers (TCAs) to realize 5th-order filter while 32, 48, 66 TCAs were respectively incorporated to achieve 3rd, 5th, 9th-order complex responses in other prior art implementations. In fact, it is found that the most efficient design among this category requires “3.7” TCAs per pole. On the other hand, first order filters may require only two devices. However, such filters would exhibit poor stopband attenuations since they are obtained from their first-order LPF counterparts. The available complex biquad filters incorporate 12 TCAs, 4 op-amps, 8 op-amps, 4 op-amps, and 10 CCIIs, depending on the chosen design. There remains the need for more efficient biquad complex filters.
Thus, an optimal low power complex filter solving the aforementioned problems is desired.